Of course, there is no reason that the Mega couldn’t cover their HQ using a hodgepodge of 800 smaller wards :-) In fact, in some ways that is stronger because an astral entity would have to get through many wards, not just one, and so would be more apt to get caught (and couldn’t flee quickly at all if having to take time to work through many layers of defense on the way out).
Although, given that wards don’t have a projection factor, i.e. you have to have a lodge (permanent or temporary) where you are creating it, I honestly doubt that many would include the entire building in wards. I’m not sure what RAW intends about ability to shape wards, but I tend to assume that they don’t have to be a cube and can be at least somewhat rectangular, so there is some ability to be fairly clever about where you place a limited number of wards to get best effect.
My theory is that buildings will tend to ward entrances (and newer buildings will be structured to funnel people through relatively few areas, to make it easier to catch it all in one to a few wards), the facilities of their security teams, executive office areas, and certain other high security areas (important research labs, for instance). Larger / richer/ more important / more vain places will also put wards around certain conference rooms, executive washrooms, and key chokepoints (astral entities can always work around a warded corridor/staircase, but a mage with a flock of spirits and a bunch of foci, or a dual-natured being, can’t so easily do this).
And then there are maybe a few others for less obvious reasons. A perk for certain jobs is having a desk in a warded area, giving you more of an illusion of privacy. It is fun / possibly smart to ward a completely boring area, so that astral intruders can’t assume that all warded areas are important! And the placement of other wards may actually fully block off an area that is not actually warded (you just can’t get there without going through a ward).